Recovery of radioactive caesium with insoluble hexacyanoferrates: problems and perspectives

Insoluble hexacyanoferrates have been used industrially for the recovery of caesium from nuclear liquid wastes because of their high affinity for caesium and their resistance to ionizing radiation. However, the sorption performances vary from one preparation to another and it is essential to define preparation methods which give reproducible chemical compositions and structures. Numerous copper, nickel and zinc hexacyanoferrates were synthesised and identified by their chemical analysis and X-ray diffraction patterns. Different sorption mechanisms from one phase to another, and from one medium to another were discerned. True ion exchange occurs only for MI2Zn3[Fe(CN)6]2 (MI = Na, K) and plays a very minor role with the other studied compounds. Otherwise, the sorption mechanism is complex and includes at least two steps: diffusion of ion pairs into the solid and formation of new solid phases, accompanied by a slow decomposition of the initial solid phase. (orig.)